1. Field of the Invention
This invention relates to a drive for a rotating-field machine, especially a synchronous machine which is supplied from a frequency converter and to a control providing self-controlled, field-oriented operation of the machine. Two AC voltage integrators of identical design are provided for forming two electric voltage signals which are each proportional to a different flux component in the rotating-field machine. Each AC voltage integrator comprises a zero controller having negative feedback for suppressing the DC component of the integrator output voltage, the feedback being connected from the output of the integrator to a summing point at the input of the integrator. A voltage proportional to a Y-voltage belonging to a flux component is connected to each integrator as an input voltage along with a voltage proportional to the corresponding phase current in a machine supply lead to compensate for the ohmic stator voltage drop. Another voltage, dependent on the phase current, for compensating for the reactive (inductive stray) voltage drop, is processed. The invention also relates to a method for operating the rotating-field machine drive.
2. Description of the Prior Art
A rotating field machine drive of this kind is described in German Auslegeschrift No. 26 35 965.
In one known rotating field machine drive, information regarding the position and magnitude of the flux vector in the machine is formed directly from the voltage and current at the terminals. In a three-phase rotating-field machine, it is sufficient to measure the Y-voltage and the phase current twice only and to process them in two AC voltage integrators of identical design connected thereto. In the known rotating-field machine drive, the main field voltage of the machine is determined first by subtracting from a Y-voltage, at a summing point in the input of each AC voltage integrator, a voltage proportional to the ohmic stator voltage drop of the rotating field machine as well as a voltage proportional to the derivative of the stator current with respect to time, to take the reactive (inductive stray) voltage drop into consideration. Subsequently, two of the three flux components in the rotating field machine are determined in the two AC voltage integrators by integration of the main field voltage. These two flux components determine the position and the magnitude of the flux vector.
Information describing the position and the magnitude of the flux vector makes it possible to operate the synchronous machine field-oriented (see Siemens-Zeitschrift 1971, pages 765 to 768 and German Pat. No. 23 53 598).
When starting the synchronous machine, it is of particular importance to determine the flux components as accurately as possible for localizing the position of the rotor. In the worst case, a measuring error can lead to a failure of the synchronous machine to start.
In the rotating field machine drive described in German Auslegeschrift No. 26 35 965, the zero-controller in the DC voltage integrator, consisting of an integrator and a proportional-integral-zero controller with negative feedback, is designed so that it can be disconnected. The zero-controller serves to prevent the output voltage of the integrator from drifting off, due to DC components of the input voltages present at the summing point.
To start the synchronous machine in the rotating field machine drive referred to, the position of the rotor at standstill is determined by first switching on the field excitation, the stator windings being initially not supplied on the converter side. The flux components can then be obtained very precisely from the voltage components induced in the stator windings, when the zero control is disconnected, since the zero drive, if it were connected, would strive to control the starting values determined by the integrator by localizing the position of the induced voltages toward zero, thus falsifying the determination of rotor position. After the stator current is switched on and the synchronous machine has been started, the zero control is then connected, and the DC components of each flux component occurring in normal operation are regulated. However, when the zero control is suddenly connected to the running machine, transients in the nature of a decaying oscillation occur in the control loop and also in the torque. This can temporarily reduce the available torque of the synchronous machine.
In addition, angle and amplitude errors occur, with the zero-controller of the above described rotating field machine drive, which depend on the frequency of the machine. The angle errors, in particular, are significant. Due to the relatively large angle errors, the excitation vector, which has a fixed relation to the flux vector, is not in the desired relationship to the flux vector, with the control concept employed, so that only a reduced component contributes to torque production. As a result, to obtain the needed torque, the converter and the rotating field machine must be designed for larger currents to provide a sufficiently large component of the excitation vector, perpendicular to the flux vector, to accommodate larger angle errors. Heretofore, the capacity of the existing rotating-field machines has been limited by the angle errors.
In addition, stability problems arise in the rotating field machine drive mentioned above, when beats occur between the network and the machine frequency; these problems limit the permissible frequency range. If, for instance, the zero control is designed so that, at an operating frequency of 100 Hz, beat frequencies of 1 Hz are sufficiently damped, then operating at a frequency of 1 Hz is impossible since this frequency is interpreted by the zero controller as an interference frequency and is attenuated beyond the permissible degree.
It is an object of the invention to provide a rotating field machine drive of the type mentioned at the outset having optimum converter and machine utilization in an enlarged speed control range, and in which problems arising from the connecting of the zero control are avoided.